By Andrei NAZARENKO, Dr. Sc. (Tech.), Program Study Center, Russian Academy of Sciences
Available statistics tell us that an annual average of about 100 man-made space objects have been orbited by various countries over the last 30 years. Combined with the bits and pieces left from all sorts of misfirings and explosions during launchings and from various assembly and disassembly operations in orbit, a total of 600 to 700 pieces of junk enter near-Earth space each year. Russian and American experts have catalogued over the aforesaid period close upon 22,000 man-made objects, ranging from 10 to 30 cm in size. According to their count some 14,500 such objects have burned down in dense lower atmosphere, but the remaining 7,500 of these objects are still hurtling over our heads. And only 5 percent of them is really useful equipment and all the rest is nothing but junk.
To make the picture even grimmer, experts tell us that the rate of junk accumulation in space is twice as high as the rate of its "self- purification". And most of the junk is orbiting the Earth at altitudes from 600 to 1,600 km - the two "belts" wherein most of the man- made satellites are operating.
Experts laid special emphasis on the excessive pollution of the geostationary orbit wherein more than 1,000 objects of different kind are concentrated. Used-up space probes can come as close as 10 km one to the other and this imposes limitations on the uses of the geostationary orbit.
The first to sound the alarm were NASA experts in the United States where a special study group was set up in 1981 from representatives of various agencies concerned. These studies were joined by the European Space Agency some ten years ago. And as for Russia, the problem has not escaped the attention of its experts who mainly come from Moscow State University, the Institute of Space Studies of the Russian Academy, the Central Research Institute of Engineering, the Central Research Institute of the Ministry of Defense, the VYMPEL Corporation, the Center for Program Research of the Russian Academy and several other agencies.
As we all know from the news, a cause of very grave international concern are used-up satellites carrying nuclear sources of power. At the end of their service life such nuclear-powered satellites are usually shifted into higher orbits (800 to 1,000 km) where they can be safely "mothballed" for hundreds of years. But there have been some truly dramatic mishaps when such satellites went out of control and "sunk" into lower atmosphere and even plunged back to Earth as was the case with the ill-fated Cosmos 954 and Cosmos 1402. In all such accidents it is the host country that must pay for any harm or damage done to anyone in keeping with international law.
Twice in the past the international public watched with anxiety reports on the falls back to the ground of big and heavy sections and fragments of used-up orbital stations - Scylab and Salute 7/Cosmos 1686.
One major, if not the main headache bothering space experts is the risk of a collision of a satellite in active service with pieces of space junk measuring from several microns to several meters in size. The consequences of such orbital collisions can range from explosions to serious structural damage and damage to solar panels and other instruments and equipment.
Contrary to what one could expect, even tiny specks of space garbage (from 8 to 10 mm in size) can cause appreciable harm. According to current assessments their number is 40 to 100 times greater than the number of catalogued space objects and amounts to several million. A very real hazard today is the chance of collisions of space probes and satellites with bits of garbage of more that 1 cm in size. For the Mir orbital station, for example, which is orbiting the Earth at an
altitude of about 400 km, the risk amounts to several percent. Given an average velocity of some 12 km/s, the force of the impact of any such projectile would be comparable to an explosion and that means that with the steadily growing probability of such collisions our space probes have to be adequately protected.
And there is one more aspect to the general problem of "space pollution". The collision of pieces of space debris can become what experts describe as an avalanche process as a result of which all space probes travelling at the same or lower altitudes from the ground could be destroyed. Any further activities in space would be made impossible until most of the bigger fragments descend into lower atmosphere and bum down. The whole process, we are told, could continue for a few decades and some experts feel that in the "busiest" orbits this process is already in progress, and in about 50 years' time the frequency of collisions is bound to increase by 10 to 20 times. This makes the task of assessing the mechanisms of any such "chain reaction" in space really important.
What should also be noted at this point is that there is no one common technique of detecting and tracking objects in space. To give just one example, the number of such objects listed in the Russian catalogue amounts to only 80 percent of the American total. This gap is explained by the fact that tracking devices within the Russian control have a much "narrower" angle of view than the American ones which are spread out over greater territory As for the authenticity of detection methods used by both sides, it is confirmed by the fact that more than 90 percent of objects in the Russian catalogue are also registered in the American one.
Having said all that, can experts try and "predict" dangerous collisions of space probes with bits of "space junk"? The answer to this one, unfortunately, is no. This is because the available instruments cannot ensure a constant watch on what are the most numerous and dangerous fragments (less than 10-30 cm). Secondly, it is difficult to measure exactly the orbits of even the catalogued objects. The only possibility is to try and use a credible statistical model of these dangerous bits and pieces. This model can be built on the basis of information about actual impacts of such fragments upon the shell and structural elements of space stations and space freighters. And it would also be necessary to stage simulation experiments which should try and anticipate the results of steadily growing space pollution.
As for the Russian side, its experts joined these studies later than their US and \\est European counterparts. But even so, the Russians have been able to work out their own effective solutions.
Until recently both Russian and Western experts used a traditional method of assessing the density of space objects and calculating collision probability which was based upon a piece-by-piece analysis of the catalogued objects. Simple algorithms were used in conjunction with powerful computers using hundreds of hours of "computer time".
Our own feeling is that the greatest progress in building "space junk" models has been made by experts from the Program Studies Center of the Russian Academy. They have developed a new statistical theory which regards, roughly speaking, a multitude of moving space objects as a kind of continuous medium. The method is so effective it can be used to obtain quick results using a conventional PC.
Finally, a look into the future. Prognostications have been made for the expected increase in the numbers of space objects from 1960 to 2004. According to these forecasts there should be as many as 5,100 of them at altitudes up to 2,000 km by 1990. The actual figure of 4,860 cited in the NORAD catalogue deviates from the forecast by no more than 5 percent, which is good accuracy By the year 2004 the number of objects at altitudes up to 2,000 km should approach 7,000, and as for long-term predictions all of them point to an irreversible growth of these numbers.
But is the future really so grim and hopeless? If our common attitude to "space junk" does not change, and change soon, the situation will certainly become worse with every passing year.
Experts have considered four scenarios of technological activities in space by members of the international community and prepared prognostications for orbits at altitudes from 200 to 2,000 km up to the year 2023. In all of these scenarios (except the one which rules out any explosions of space probes and provides for reduced numbers of launchings) the total number of man-made objects in space continues to grow. And that simply means that in order to stabilize the amounts of "space junk" we have to reduce the number of launchings and take steps to increase the life-span of our satellites and probes.
But even so, we are told, the levels of pollution of near-Earth space at altitudes over 1,100 km will not be reduced in any appreciable degree. What has to be done is to try and bring down to Earth "discarded" space probes and take steps to prevent their disintegration into fragments, both accidental and intentional. Steps must also be taken to minimize the number of any "detachable" parts, such as the last stages of carrier rockets and space boosters. One encouraging example of this strategy is offered by the giant Energia space carrier whose last stage is not left orbiting the Earth.
A far more challenging task consists in "clearing" orbits from junk. At the present stage experts suggest, for example, using Buran and/or Shuttle missions for clearing orbits from junk, developing a kind of space tugs fitted with mechanical arms and suchlike ideas. Looking simple enough at first sight, all these methods will be fairly expensive and energy-consuming, to say nothing of the amount of additional studies and tests they involve.
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